Image sensor and electronic camera

ABSTRACT

An image sensor includes a pixel, the pixel has: a first photoelectric conversion unit and a second photoelectric conversion unit that photoelectrically convert light to generate a charge; an accumulation unit that accumulates at least one of the charge generated by the first photoelectric conversion unit and the charge generated by the second photoelectric conversion unit; a first output unit and a second output unit that output a signal based on the charge accumulated in the accumulation unit; and an adjustment unit that adjusts a capacitance of the accumulation unit if a signal based on the charges generated by the first photoelectric conversion unit and the second photoelectric conversion unit is output from the first output unit.

TECHNICAL FIELD

The present invention relates to an image sensor and an electronic camera.

BACKGROUND ART

PTL1 discloses an image-capturing device that adds (mixes) charges of two light receiving elements and reads out a potential corresponding to the added charge. However, the conventional image-capturing device has a problem that a conversion gain in converting the added charge into the potential is low.

CITATION LIST Patent Literature

-   PTL1: Japanese Laid-Open Patent Publication No. 2016-139859

SUMMARY OF INVENTION

An image sensor according to the 1st aspect of the present invention comprises a pixel, the pixel has: a first photoelectric conversion unit and a second photoelectric conversion unit that photoelectrically convert light to generate a charge; an accumulation unit that accumulates at least one of the charge generated by the first photoelectric conversion unit and the charge generated by the second photoelectric conversion unit; a first output unit and a second output unit that output a signal based on the charge accumulated in the accumulation unit; and an adjustment unit that adjusts a capacitance of the accumulation unit if a signal based on the charges generated by the first photoelectric conversion unit and the second photoelectric conversion unit is output from the first output unit.

An image sensor according to the 2nd aspect of the present invention comprises a pixel, the pixel has: a first photoelectric conversion unit and a second photoelectric conversion unit that photoelectrically convert light to generate a charge; a first accumulation unit that accumulates the charge generated by the first photoelectric conversion unit; a second accumulation unit that accumulates the charge generated by the second photoelectric conversion unit; a first output unit that is connected to the first accumulation unit via a first connection unit; and a second output unit that is connected to the second accumulation unit via a second connection unit.

An electronic camera according to the 3rd aspect of the present invention comprises: the image sensor according to the 1st aspect or the 2nd aspect; and an image generation unit that generates image data based on a signal output from the image sensor.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of an image-capturing device according to a first embodiment.

FIG. 2 is a circuit diagram showing a configuration of a pixel according to the first embodiment.

FIG. 3 is a view for explaining an example of an operation of the image sensor according to the first embodiment.

FIG. 4 is a view for explaining another example of an operation of the image sensor according to the first embodiment.

FIG. 5 is a circuit diagram showing a configuration of a part of the image sensor according to the first embodiment.

FIG. 6 is a timing chart showing an example of an operation of the image sensor according to the first embodiment.

FIG. 7 is a timing chart showing another example of an operation of the image sensor according to the first embodiment.

FIG. 8 is a timing chart showing another example of an operation of the image sensor according to the first embodiment.

FIG. 9 is a view for explaining an example of an operation of the image sensor according to a first modification.

FIG. 10 is a circuit diagram showing a configuration of a pixel according to a second modification.

DESCRIPTION OF EMBODIMENTS

First Embodiment

FIG. 1 is a block diagram showing a configuration of an image-capturing device according to a first embodiment. FIG. 1 shows an example of a configuration of an electronic camera 1 (hereinafter referred to as a camera 1), which is an example of an image-capturing device according to the first embodiment. The camera 1 includes an image-capturing optical system (image-forming optical system) 2, an image sensor 3, a control unit 4, a memory 5, a display unit 6, and an operation unit 7. The image-capturing optical system 2 has a plurality of lenses, including a focus adjustment lens (focus lens), and a diaphragm and forms a subject image on the image sensor 3. Note that the image-capturing optical system 2 may be detachable from the camera 1.

The image sensor 3 is, for example, a CMOS image sensor. The image sensor 3 receives light flux having passed through an exit pupil of the image-capturing optical system 2 and captures a subject image. As will be described later in detail, the image sensor 3 has a plurality of pixels arranged two-dimensionally (in a row direction and in a column direction intersecting the row direction). Each pixel has a microlens and a plurality of photoelectric conversion units (for example, two photoelectric conversion units). The photoelectric conversion unit includes, for example, a photodiode (PD). The image sensor 3 photoelectrically converts incident light to generate a signal, and outputs the generated signal to the control unit 4.

As will be described later in detail, the image sensor 3 outputs a signal for generating image data (i.e., an image-capturing signal) and a pair of focus detection signals for performing phase difference type focus detection for the focus of the image-capturing optical system 2 (i.e., first and second focus detection signals) to the control unit 4. The first and second focus detection signals are, respectively, signals obtained by photoelectrically converting first and second images based on first and second light fluxes passing through first and second regions of the exit pupil of the image-capturing optical system 2.

The memory 5 is, for example, a recording medium such as a memory card. The memory 5 records image data and the like. Data writing to the memory 5 and data reading from the memory 5 are performed by the control unit 4. The display unit 6 displays an image based on image data, information on photographing such as a shutter speed and an aperture value, and a menu screen. The operation unit 7 includes various setting switches such as a release button and a power switch, and outputs an operation signal corresponding to each operation to the control unit 4.

The control unit 4 includes a CPU, a ROM, a RAM, and the like, and controls each unit of the camera 1 based on a control program. The control unit 4 includes an image data generation unit 4 a and a focus detection unit 4 b. The image data generation unit 4 a performs various types of image processing on the image-capturing signal output from the image sensor 3 to generate image data. The image processing includes, for example, known image processing such as tone conversion processing, color interpolation processing, and edge enhancement processing.

The focus detection unit 4 b performs a focus detection process necessary for automatic focus adjustment (AF) of the image-capturing optical system 2 by a known phase difference detection scheme. Specifically, the focus detection unit 4 b detects an image shift amount between the first and second images based on a pair of focus detection signals output from the image sensor 3, and calculates a defocus amount based on the detected image shift amount. The focus adjustment is automatically performed by driving the focus adjustment lens in accordance with the defocus amount.

The control unit 4 preforms a process of individually reading out signals from the plurality of photoelectric conversion units of each pixel of the image sensor 3 (a first control mode) and a process of adding and reading out signals from the plurality of photoelectric conversion units (a second control mode). In this embodiment, in the first control mode, a signal based on a charge generated by the first photoelectric conversion unit and a signal based on a charge generated by the second photoelectric conversion unit are individually (i.e., independently) read out as a pair of focus detection signals, as will be described later in detail.

In the second control mode, the control unit 4 performs a process of adding the signals from the first and second photoelectric conversion units to read out the processed signal as an image-capturing signal. Here, the “process of adding” includes a process of averaging a plurality of signals, a process of weighting and adding a plurality of signals, and the like. The control unit 4 performs the first control mode in a case of performing a phase difference AF to read out a pair of focus detection signals from the image sensor 3, and performs the second control mode in a case of generating image data to read out an image-capturing signal from the image sensor 3.

FIG. 2 is a circuit diagram showing a configuration of a pixel of the image sensor 3 according to the first embodiment. The pixel 10 includes a microlens ML, a first photoelectric conversion unit 11 a, a second photoelectric conversion unit 11 b, a first transfer unit 12 a, a second transfer unit 12 b, and a first reset unit 13 a, a second reset unit 13 b, a first accumulation unit 14 a, and a second accumulation unit 14 b. The pixel 10 further includes a first amplification unit 15 a, a second amplification unit 15 b, a first selection unit 16 a, a second selection unit 16 b, an addition switch unit 17, a first connection switch unit 19 a, a second connection switch unit 19 b, a first region 20 a, and a second region 20 b.

The microlens ML focuses light, which is incident through the image-capturing optical system 2 in FIG. 1, on the first photoelectric conversion unit 11 a and the second photoelectric conversion unit 11 b. Note that the microlens ML is denoted by an oval-shaped line that surrounds the first and second photoelectric conversion units 11 a, 11 b to indicate that a light flux having passed through the microlens ML is incident on the first and second photoelectric conversion units 11 a, 11 b. The oval shape does not denote the actual size or actual shape of the microlens ML.

The first photoelectric conversion unit 11 a and the second photoelectric conversion unit 11 b are photodiodes PD (PDa, PDb), which have a function of converting incident light into charges and accumulating the charges obtained through photoelectric conversion. The first photoelectric conversion unit 11 a and the second photoelectric conversion unit 11 b are arranged so as to correspond to one microlens ML, and receive light fluxes that have passed through mutually different regions of the exit pupil of the image-capturing optical system 2. That is, the first photoelectric conversion unit 11 a and the second photoelectric conversion unit 11 b, respectively, photoelectrically convert first and second images based on first and second light fluxes passing through first and second regions of the exit pupil of the image-capturing optical system 2.

The first transfer unit 12 a includes a transistor M1 a, which is controlled by a signal TX1. When a transistor M9 a of the first connection switch unit 19 a is off, the first transfer unit 12 a transfers a charge obtained through photoelectric conversion by the first photoelectric conversion unit 11 a to the first accumulation unit 14 a. That is, the first transfer unit 12 a forms a charge transfer path between the first photoelectric conversion unit 11 a and the first accumulation unit 14 a. On the other hand, when the transistor M9 a of the first connection switch unit 19 a is on, the first transfer unit 12 a transfers the charge obtained through photoelectric conversion by the first photoelectric conversion unit 11 a to the first accumulation unit 14 a and the first region 20 a. The transistor M1 a is a first transfer transistor. Capacitances such as a capacitance (parasitic capacitance) of each transistor connected to the first accumulation unit 14 a and a wiring capacitance are added to the first accumulation unit 14 a. The first accumulation unit 14 a accumulates (holds) the charge transferred to the first accumulation unit 14 a, and converts the charge into a voltage divided by a capacitance value.

The first connection switch unit 19 a includes a transistor M9 a, which is controlled by a signal ADD_FD1, and connects (couples) the first accumulation unit 14 a to the first amplification unit 15 a. It can also be said that the first connection switch unit 19 a connects the first accumulation unit 14 a to the first region 20 a. Capacitances such as a capacitance of each transistor (e.g., a gate capacitance of the first amplification unit 15 a connected to the first region 20 a) and a wiring capacitance are added to the first region 20 a. The first region 20 a accumulates the charge transferred to the first region 20 a, and converts the charge into a voltage divided by a capacitance value.

The first amplification unit 15 a amplifies and outputs a signal based on the charge transferred from the photoelectric conversion unit. The first amplification unit 15 a includes a transistor M3 a. A drain (terminal) of the transistor M3 a is connected to a power supply VDD, and a gate (terminal) of the transistor M3 a is connected to a drain of the transistor M9 a of the first connection switch unit 19 a. A source (terminal) of the transistor M3 a is connected to a transistor M4 a of the first selection unit 16 a. The source of the transistor M3 a of the first amplification unit 15 a is connected to a first vertical signal line VLa via the first selection unit 16 a. The first amplification unit 15 a functions as a part of a source follower circuit, with a first current source 25 a shown in FIG. 3 as a load current source. The transistor M3 a is a first amplification transistor.

The first reset unit 13 a includes a transistor M2 a, which is controlled by a signal RS1, and resets charges of the first region 20 a and the first accumulation unit 14 a and resets voltages of the first region 20 a and the first accumulation unit 14 a. The transistor M2 a is a first reset transistor. The first selection unit 16 a includes a transistor M4 a, which is controlled by a signal SEL1, and outputs the signal from the first amplification unit 15 a to the first vertical signal line VLa. The transistor M4 a is a first selection transistor. The first output unit according to the present embodiment includes the first amplification unit 15 a, the first selection unit 16 a, and the first reset unit 13 a, and generates and outputs a signal based on the charge generated by the first photoelectric conversion unit 11 a.

The second transfer unit 12 b includes a transistor M1 b, which is controlled by a signal TX2. When a transistor M9 b of the second connection switch unit 19 b is off, the second transfer unit 12 b transfers a charge obtained through photoelectric conversion by the second photoelectric conversion unit 11 b to the second accumulation unit 14 b. That is, the second transfer unit 12 b forms a charge transfer path between the second photoelectric conversion unit 11 b and the second accumulation unit 14 b. On the other hand, when the transistor M9 b of the second connection switch unit 19 b is on, the second transfer unit 12 b transfers the charge obtained through photoelectric conversion by the second photoelectric conversion unit 11 b to the second accumulation unit 14 b and the second region 20 b. The transistor M1 b is a second transfer transistor. Capacitances such as a capacitance of each transistor connected to the second accumulation unit 14 b and a wiring capacitance are added to the second accumulation unit 14 b. The second accumulation unit 14 b accumulates (holds) the charge transferred to the second accumulation unit 14 b, and converts the charge into a voltage divided by a capacitance value.

The second connection switch unit 19 b includes a transistor M9 b, which is controlled by a signal ADD_FD2, and connects (couples) the second accumulation unit 14 b to the second amplification unit 15 b. It can also be said that the second connection switch unit 19 b connects the second accumulation unit 14 b to the second region 20 b. Capacitances such as a capacitance of each transistor (e.g., a gate capacitance of the second amplification unit 15 b connected to the second region 20 b) and a wiring capacitance are added to the second region 20 b. The second region 20 b accumulates the charge transferred to the second region 20 b, and converts the charge into a voltage divided by a capacitance value.

The second amplification unit 15 b amplifies and outputs a signal based on the charge transferred from the photoelectric conversion unit. The second amplification unit 15 b includes a transistor M3 b. A drain of the transistor M3 b is connected to a power supply VDD, and a gate of the transistor M3 b is connected to a drain of the transistor M9 b of the second connection switch unit 19 b. A source of the transistor M3 b is connected to a transistor M4 b of the second selection unit 16 b. The source of the second amplification unit 15 b is connected to a second vertical signal line VLb via the second selection unit 16 b. The second amplification unit 15 b functions as a part of a source follower circuit, with a second current source 25 b shown in FIG. 3 as a load current source. The transistor M3 b is a second amplification transistor.

The second reset unit 13 b includes a transistor M2 b, which is controlled by a signal RS2, and resets charges of the second region 20 b and the second accumulation unit 14 b and resets voltages of the second region 20 b and the second accumulation unit 14 b. The transistor M2 b is a second reset transistor. The second selection unit 16 b includes a transistor M4 b, which is controlled by a signal SEL2, and outputs the signal from the second amplification unit 15 b to the second vertical signal line VLb. The transistor M4 b is a second selection transistor. The second output unit according to the present embodiment includes the second amplification unit 15 b, the second selection unit 16 b, and the second reset unit 13 b, and generates and outputs a signal based on the charge generated by the second photoelectric conversion unit 11 b.

The addition switch unit 17 includes a transistor M7, which is controlled by a signal ADD_FD3, and connects the first accumulation unit 14 a to the second accumulation unit 14 b. It can also be said that the addition switch unit 17 connects the first transfer unit 12 a to the second transfer unit 12 b.

In the first control mode, the control unit 4 controls the image sensor 3 so that the transistor M7 of the addition switch unit 17 is off (in an off state), the transistor M9 a of the first connection switch unit 19 a is on, and the transistor M9 b of the second connection switch unit 19 b is on. The charge obtained through photoelectric conversion by the first photoelectric conversion unit 11 a is transferred to the first accumulation unit 14 a and the first region 20 a. Then, a signal (first pixel signal) according to the charge transferred from the first photoelectric conversion unit 11 a is read out to a first vertical signal line VLa by the first amplification unit 15 a and the first selection unit 16 a. Further, the charge obtained through photoelectric conversion by the second photoelectric conversion unit 11 b is transferred to the second accumulation unit 14 b and the second region 20 b. A signal (second pixel signal) according to the charge transferred from the second photoelectric conversion unit 11 b is read out to a second vertical signal line VLb by the second amplification unit 15 b and the second selection unit 16 b.

Thus, in the first control mode, the first pixel signal generated in accordance with the charge from the first photoelectric conversion unit 11 a is output to the first vertical signal line VLa, and the second pixel signal generated in accordance with the charge from the second photoelectric conversion unit 11 b is output to the second vertical signal line VLb. The first pixel signal and the second pixel signal are output to the control unit 4 as a pair of focus detection signals after being subjected to signal processing by a column circuit or the like described later.

Next, a basic operation of the second control mode will be described. In the second control mode, the control unit 4 sets the transistor M7 of the addition switch unit 17 to be on. For example, the control unit 4 sets the transistor M9 a of the first connection switch unit 19 a to be on, the transistor M9 b of the second connection switch unit 19 b to be off, the transistor M4 a of the first selection unit 16 a to be on, and the transistor M4 b of the second selection unit 16 b to be off. The charges obtained through photoelectric conversion by the first and second photoelectric conversion units 11 a, 11 b, respectively, are transferred by the first and second transfer units 12 a, 12 b and added by the addition switch unit 17. An addition pixel signal is generated by the first amplification unit 15 a and the first selection unit 16 a in accordance with the added charge, and the addition pixel signal is read out to the first vertical signal line VLa. Note that when the transistor M9 a of the first selection unit 19 a is off, the transistor M9 b of the second selection unit 19 b is on, the transistor M4 a of the first connection switch unit 16 a is off, and the transistor M4 b of the second connection switch unit 16 b is on, the addition pixel signals are read out to the second vertical signal line VLb.

In the present embodiment, the second control mode involves a one-row reading scheme for reading out a signal for each one row of the pixels 10 arranged two-dimensionally, and a two-row simultaneous reading scheme for reading out signals for two rows simultaneously. Hereinafter, the “one-row reading scheme” of the second control mode will be described with reference to FIG. 3, and the “two-row simultaneous reading scheme” of the second control mode will be described with reference to FIG. 4.

FIG. 3 shows one pixel column of the plurality of pixels 10 arranged two-dimensionally. In the image sensor 3, the first vertical signal line VLa and the second vertical signal line VLb are provided for a column of the plurality of the pixels 10 arranged in the column direction, that is, in the vertical direction. Additionally, a first current source 25 a and a first column circuit unit 40 a are provided for the first vertical signal line VLa, and a second current source 25 b and a second column circuit unit 40 b are provided for the second vertical signal line VLb. Note that in the example shown in FIG. 3, for simplification of description, only one pixel in the row direction by three pixels in the column direction in the pixels 10 is shown; however, the image sensor 3 has, for example, several millions to several hundreds of millions of pixels or more.

The first current source 25 a is connected to each pixel 10 via the first vertical signal line VLa, and the second current source 25 b is connected to each pixel 10 via the second vertical signal line VLb. The first current source 25 a and the second current source 25 b generate current for reading out a signal from each pixel 10. The first current source 25 a supplies the generated current to the first vertical signal line VLa and the first selection unit 16 a and the first amplification unit 15 a of each pixel 10. Similarly, the second current source 25 b supplies the generated current to the second vertical signal line VLb and the second selection unit 16 b and the second amplification unit 15 b of each pixel 10.

Each of the first column circuit unit 40 a and the second column circuit unit 40 b is configured to include an analog/digital conversion unit (AD conversion unit). The first column circuit unit 40 a converts a signal input from each pixel 10 through the first vertical signal line VLa into a digital signal. The second column circuit unit 40 b converts a signal input from each pixel 10 through the second vertical signal line VLb into a digital signal. The first column circuit unit 40 a and the second column circuit unit 40 b output the converted digital signal to a horizontal transfer unit described later.

In the one-row reading scheme of the second control mode, the image sensor 3 reads out a signal (addition pixel signal) according to a charge obtained by adding the charge of the first photoelectric conversion unit 11 a and the charge of the second photoelectric conversion unit 11 b, for example, to the first vertical signal line VLa. In the example shown in FIG. 3, an addition pixel signal is read out from the pixel 10 in the first row, that is, the pixel 10 in the lowermost row. In the pixel 10 in the first row, the transistor M7 of the addition switch unit 17 is on, the transistor M9 a of the first connection switch unit 19 a is on, and the transistor M9 b of the second connection switch unit 19 b is off. Further, in the pixel 10 in the first row, the transistor M4 a of the first selection unit 16 a is on, and the transistor M4 b of the second selection unit 16 b is off. On the other hand, in the pixel 10 in other rows such as second and third rows, the transistors M4 a, M4 b of the first and second selection units 16 a, 16 b are off. Note that in FIG. 3, ON indicates that a transistor is on (in a connected state, in a conductive state, in a short circuited state) and OFF indicates that a transistor is off (in a disconnected state, in a nonconductive state, in an open state, in a disconnected state).

In the pixel 10 in the first row, the transistor M7 of the addition switch unit 17 is turned on and the transistor M9 a of the first connection switch unit 19 a is turned on, so that the first and second accumulation units 14 a, 14 b and the first region 20 a are electrically connected to each other. Additionally, the first transfer unit 12 a and the second transfer unit 12 b are electrically connected to each other. Since the transistor M9 b of the second connection switch unit 19 b is off, the second region 20 b is electrically separated from the first accumulation unit 14 a, the second accumulation unit 14 b, and the first region 20 a. This allows the charge transferred from the first photoelectric conversion unit 11 a and the charge transferred from the second photoelectric conversion unit 11 b to be added in the first accumulation unit 14 a, the second accumulation unit 14 b, and the first region 20 a. It can also be said that the charge generated by the first photoelectric conversion unit 11 a and the charge of the second photoelectric conversion unit 11 b are mixed (combined).

Capacitances of the first accumulation unit 14 a, the second accumulation unit 14 b, and the first region 20 a are electrically connected to one another, so that the charges transferred from the first photoelectric conversion unit 11 a and the second photoelectric conversion unit 11 b are distributed to the capacitances. A voltage of the first accumulation unit 14 a, a voltage of the second accumulation unit 14 b, and a voltage of the first region 20 a are averaged, and the averaged voltage is input to the first amplification unit 15 a. That is, the first amplification unit 15 a receives a voltage that is a result of dividing a charge, which is a obtained by an addition of the accumulated charges of the first and second accumulation units 14 a, 14 b and the first region 20 a, by a combined capacitance value of the capacitances of the first and second accumulation units 14 a, 14 b and the first region 20 a.

In the pixel 10 in the first row, the transistor M4 a of the first selection unit 16 a is turned on, so that the first amplification unit 15 a receives a current from the first current source 25 a to operate in a saturation region. Since the transistor M3 a of the first amplification unit 15 a operates in the saturation region, a gate capacitance of the first amplification unit 15 a has a generally constant capacitance value. Since the gate capacitance of the first amplification unit 15 a has a generally constant value, the combined capacitance of the capacitance of the first accumulation unit 14 a, the capacitance of the second accumulation unit 14 b, and the capacitance of the first region 20 a also keeps a predetermined value. The first amplification unit 15 a generates an addition pixel signal based on a voltage that is a result of dividing a charge, which is obtained by an addition of the accumulated charges of the first and second accumulation units 14 a, 14 b and the first region 20 a, by a combined capacitance value. The addition pixel signal is sent to the first vertical signal line VLa via the first selection unit 16 a.

As described above, after the addition pixel signals is read out from the pixel 10 in the first row to the first vertical signal line VLa, pixels 10 in the second row, the third row, and so on are sequentially selected row by row in the image sensor 3, and addition pixel signals are read out from the pixels 10 to the first vertical signal line VLa. The addition pixel signal of the pixel 10 output to the first vertical signal line VLa is converted into a digital signal by the first column circuit unit 40 a, and then output to the control unit 4 as an image-capturing signal.

In the example shown in FIG. 3, the addition pixel signals of the pixels 10 in each row are read out to the first vertical signal line VLa. The image sensor 3 can therefore stop the generation of current by the second current source 25 b connected to the second vertical signal line VLb to which the addition pixel signal is not read out, so that a power consumption of the image sensor 3 can be reduced. Note that when the transistor M4 a of the first selection unit 16 a is off, the transistor M4 b of the second selection unit 16 b is on, the transistor M9 a of the first connection switch unit 19 a is off, and the transistor M9 b of the second connection switch unit 19 b is on, the addition pixel signals can be read out from the pixels 10 to the second vertical signal line VLb.

In the present embodiment, a conversion gain when the first and second accumulation units 14 a, 14 b and the first region 20 a convert charges into voltage is the inverse of a combined capacitance value of the capacitance of the first accumulation unit 14 a, the capacitance of the second accumulation unit 14 b, and the capacitance of the first region 20 a. The conversion gain can therefore be increased, compared with a case where the second region 20 b is electrically connected to the first and second accumulation units 14 a, 14 b and the first region 20 a. Consequently, a S/N ratio can be improved to obtain an addition pixel signal having a lower noise.

Further, in the present embodiment, a conversion gain when the charges are converted into voltage, i.e., the inverse of a combined capacitance value of the capacitance of the first accumulation unit 14 a, the capacitance of the second accumulation unit 14 b, and the capacitance of the first region 20 a is always generally constant. The addition pixel signal therefore depends on the accumulated charges of the first and second accumulation units 14 a, 14 b and the first region 20 a, and has a high linearity. The following description presents that the conversion gain of the first and second accumulation units 14 a, 14 b and the first region 20 a is high and that the conversion gain is always generally constant, in comparison with a comparative example.

In the present embodiment, the transistor M9 b of the second connection switch unit 19 b is turned off, so that the second region 20 b is electrically disconnected from the first and second accumulation units 14 a, 14 b and the first region 20 a, as described above. Thus, a capacitance of a region to which the charge is transferred from the first and second photoelectric conversion units 11 a, 11 b is reduced. That is, the charges generated by the first and second photoelectric conversion units 11 a, 11 b are not accumulated on the capacitance of the second region 20 b, but are accumulated on the capacitances of the first and second accumulation units 14 a, 14 b and the first regions 20 a. Consequently, the conversion gain of the charge voltage can be increased.

Further, in the present embodiment, a gate capacitance of the transistor M3 b of the second amplification unit 15 b is electrically disconnected from the first and second accumulation units 14 a, 14 b and the first region 20 a. The conversion gain when changing the transferred charge to voltage is, therefore, the inverse of the combined capacitance value of the capacitance of the first accumulation unit 14 a, the capacitance of the second accumulation unit 14 b, and the capacitance of the first region 20 a. Thus, the conversion gain has a generally constant value without being affected by fluctuation of the gate capacitance of the transistor M3 b.

On the other hand, in the comparative example, the first connection switch unit 19 a and the second connection switch unit 19 b are omitted from the pixel 10 of FIG. 3. In this case, the first accumulation unit 14 a and the second accumulation unit 14 b and the first region 20 a and the second region 20 b are electrically connected to one another. The first amplification unit 15 a receives a voltage that is a result of dividing a charge, which is obtained by adding the accumulated charges of the first and second accumulation units 14 a, 14 b and the first and second regions 20 a, 20 b, by a combined capacitance value of the capacitances of the first and second accumulation units 14 a, 14 b and the first and the second regions 20 a, 20 b. The conversion gain of the first and second accumulation units 14 a, 14 b and the first and second regions 20 a, 20 b is the inverse of a combined capacitance value of the capacitance of the first accumulation unit 14 a, the capacitance of the second accumulation unit 14 b, the capacitance of the first region 20 a, and the capacitance of the second region 20 b. As a result, the conversion gain is reduced so that the S/N ratio is reduced.

Also, when the transistor M4 a of the first selection unit 16 a is turned on and the transistor M4 b of the second selection unit 16 b is turned off, a current is supplied to the first amplification unit 15 a while no current is supplied to the second amplification unit 15 b, so that the second amplification unit 15 b operates in a weak inversion region. In the operation in the weak inversion region, the gate capacitance of the second amplification unit 15 b changes in accordance with a signal input to the gate. In the comparative example, due to the fluctuation of the gate capacitance of the second amplification unit 15 b, the combined capacitance added to the first amplification unit 15 a also fluctuates. This leads to a fluctuation of the conversion gain of the charge voltage.

FIG. 4 is a diagram for explaining the two-row simultaneous reading scheme of the second control mode. The two-row simultaneous readout scheme of the second control mode involves, for pixels in two rows, reading out an addition pixel signal from one pixel in one row to the first vertical signal line VLa and, at the same time, reading out an addition pixel signal from another pixel in the other row to the second vertical signal line VLb. Details will be described below.

In the pixel 10 in the lowermost row, the transistor M7 of the addition switch unit 17 is on, the transistor M9 a of the first connection switch unit 19 a is on, and the transistor M9 b of the second connection switch unit 19 b is off. Further, in the pixel 10 in the first row, the transistor M4 a of the first selection unit 16 a is on, and the transistor M4 b of the second selection unit 16 b is off. In the pixel 10 in the second row, the transistor M7 of the addition switch unit 17 is on, the transistor M9 a of the first connection switch unit 19 a is off, and the transistor M9 b of the second connection switch unit 19 b is on. Further, in the pixel 10 in the second row, the transistor M4 a of the first selection unit 16 a is off, and the transistor M4 b of the second selection unit 16 b is on.

In each of the pixels 10 in the first and second rows, since the transistor M7 of the addition switch unit 17 is in the on state, the charge transferred from the first photoelectric conversion unit 11 a and the charge transferred from the second photoelectric conversion unit 11 b are added. Further, in each of the pixels 10 in the first row, when the transistor M9 b of the second connection switch unit 19 b is turned off, the second region 20 b is electrically disconnected from the second accumulation unit 14 b. On the other hand, in each of the pixels 10 in the second row, when the transistor M9 a of the first connection switch unit 19 a is turned off, the first region 20 a is electrically disconnected from the first accumulation unit 14 a. Thus, both in the first row and in the second row, the conversion gain of the charge voltage is increased and the conversion gain has a generally constant value.

An addition pixel signal based on the charges added from the pixels 10 in the first row is read out to the first vertical signal line VLa and, at the same time, an addition pixel signal based on the charges added from the pixels 10 in the second row is read out to the second vertical signal line VLb. After the simultaneous readout from the pixels in the first row and in the second row is completed, a simultaneous readout from pixels in a third row and in a fourth row is performed and further simultaneous readouts from pixels in an odd number row and an even number row which are adjacent to each other are performed sequentially.

In this way, in the two-row simultaneous readout scheme shown in FIG. 4, addition pixel signals of pixels in two rows can be simultaneously read out. A signal can thus be read out at a high speed from each pixel 10 arranged in the image sensor 3.

A more detailed circuit configuration and operation of the image sensor 3 according to the first embodiment will be described with reference to FIGS. 5 to 8. FIG. 5 is a circuit diagram two-dimensionally showing the pixels 10 of the image sensor 3 according to the first embodiment and also showing a more detailed circuit configuration. FIG. 6 is a timing chart showing an example of an operation of the image sensor 3 in a case of the first control mode. FIG. 7 is a timing chart showing an example of an operation of the image sensor 3 in a case of the one-row reading scheme of the second control mode. FIG. 8 is a timing chart showing an example of an operation of the image sensor 3 in a case of the two-row simultaneous reading scheme of the second control mode.

As shown in FIG. 5, the image sensor 3 includes a plurality of pixels 10 arranged in a matrix, first current sources 25 a (first current source 25 a 1 to first current source 25 a 3), and second current sources 25 b (second current source 25 b 1 to second current source 25 b 3). The image sensor 3 further includes first current control units 30 a (first current control unit 30 a 1 to first current control unit 30 a 3) and second current control units 30 b (second current control unit 30 b 1 to second current control unit 30 b 3). The image sensor 3 further includes first column circuit units 40 a (first column circuit unit 40 a 1 to first column circuit unit 40 a 3), second column circuit units 40 b (second column circuit unit 40 b 1 to a second column circuit unit 40 b 3), a vertical drive unit 50, and a horizontal transfer unit 60.

The first vertical signal lines VLa (first vertical signal line VLa1 to first vertical signal line VLa3) and the second vertical signal lines VLb (second vertical signal lines VLb1 to second vertical signal line VLb3) are provided so as to correspond to each column of the pixels 10. A first current source 25 a, a first current control unit 30 a, and a first column circuit unit 40 a are provided for the first vertical signal line VLa. Further, a second current source 25 b, a second current control unit 30 b, and a second column circuit unit 40 b are provided for the second vertical signal line VLb. Note that in the example shown in FIG. 5, for simplification of description, only one pixel in the row direction by three pixels in the column direction in the pixels 10 is shown.

The vertical drive unit 50 supplies a signal TX, a signal RS, a signal SEL1, a signal SEL2, a signal ADD_FD1, a signal ADD_FD2, and a signal ADD_FD3 to each pixel 10 to control the pixel 10. The first current control unit 30 a includes switch units 31 a, 32 a and an inverter unit 33 a. The second current control unit 30 b includes switch units 31 b, 32 b and an inverter unit 33 b. The vertical drive unit 50 supplies a signal CS1_EN, a signal CS2_EN, and a voltage Vclip to the first current control unit 30 a and the second current control unit 30 b. Note that in the example shown in FIG. 5, the first transfer unit 12 a and the second transfer unit 12 b are controlled by the same signal TX, and the first reset unit 13 a and the second reset unit 13 b are controlled by the same signal RS.

The horizontal transfer unit 60 sequentially transfers digital signals converted by the first column circuit unit 40 a and the second column circuit unit 40 b to a signal processing unit (not shown). The signal processing unit performs signal processing, such as correlated double sampling and a process of correcting the signal amount, on the signal input from the horizontal transfer unit 60, and outputs the processed signal to the control unit 4 of the camera 1.

In a timing chart shown in FIG. 6, the horizontal axis indicates time and also control signals input to units of the image sensor 3 in FIG. 5 in the case of the first control mode. Further, in FIG. 6, a transistor to which the control signal is input is turned on if the control signal is at high level (e.g., at a power supply potential), and the control signal is input is turned off if the control signal is at low level (e.g., at a ground potential).

The vertical drive unit 50 sets signals ADD_FD1<2> to ADD_FD1<0> to high level, signals ADD_FD2<2> to ADD_FD2<0> to high level, and signals ADD_FD3<2> to ADD_FD3<0> to low level, in order to set the first control mode. The signal ADD_FD1 is set to high level, so that the first accumulation unit 14 a and the first amplification unit 15 a of each pixel 10 are electrically connected to each other. The signal ADD_FD2 is set to high level, so that the second accumulation unit 14 b and the second amplification unit 15 b of each pixel 10 are electrically connected to each other. Additionally, the signal ADD_FD3 is set to low level, so that the first accumulation unit 14 a and the first accumulation unit 14 b of each pixel 10 are electrically disconnected from each other.

The vertical drive unit 50 sets the signal CS1_EN to high level and also sets the signal CS2_EN to high level. The signal CS1_EN is set to high level, so that the switch unit 31 a of the first current control unit 30 a is turned on. Thus, a current is supplied from the first current source 25 a to the first vertical signal line VLa via the switch unit 31 a. Additionally, the signal CS2_EN is set to high level, so that the switch unit 31 b of the second current control unit 30 b is turned on. Thereby, the current is supplied from the second current source 25 b to the second vertical signal line VLb via the switch unit 31 b.

At time t1 shown in FIG. 6, the signal RS<2> is set to high level, so that the transistors M2 a, M2 b of the first and second reset units 13 a, 13 b are turned on in each of the pixels 10 in the uppermost row. Potentials of the first accumulation unit 14 a, the second accumulation unit 14 b, the first region 20 a, and the second region 20 b become reset potentials.

Further, at time t1, the signal SEL1<2> is set to high level, so that a signal based on the reset potential is output to the first vertical signal line VLa by the first amplification unit 15 a and the first selection unit 16 a. That is, a signal (first noise signal) generated when the potentials of the first accumulation unit 14 a and the first region 20 a are reset to the reset potentials is read out to the first vertical signal line VLa. The first noise signal from each pixel 10 output to the first vertical signal line VLa is input to the first column circuit units 40 a 1 to 40 a 3 and converted to digital signals. Further, at time t1, the signal SEL2<2> is set to high level, so that a signal (second noise signal) generated when the potentials of the second accumulation unit 14 b and the second region 20 b are reset to the reset potentials is read out to the second vertical signal line VLb by the second amplification unit 15 b and the second selection unit 16 b. The second noise signal from each pixel 10 output to the second vertical signal line VLb is input to the second column circuit units 40 b 1 to 40 b 3 and converted to digital signals.

At time t2, the signal TX<2> is set to high level, so that the transistors M1 a, M1 b of the first and second transfer units 12 a, 12 b are turned on. Thus, the charge obtained through photoelectric conversion by the first photoelectric conversion unit 11 a is transferred to the first accumulation unit 14 a and the first region 20 a. Further, the charge obtained through photoelectric conversion by the second photoelectric conversion unit 11 b is transferred to the second accumulation unit 14 b and the second region 20 b.

Further, at time t2, since the signal SEL1<2> is set to high level, a first pixel signal is output to the first vertical signal line VLa by the first amplification unit 15 a and the first selection unit 16 a. The first pixel signal from each pixel 10 output to the first vertical signal line VLa is input to the first column circuit units 40 a 1 to 40 a 3 and converted to digital signals. Further, at time t2, since the signal SEL2<2> is set to high level, a second pixel signal is output to the second vertical signal line VLb by the second amplification unit 15 b and the second selection unit 16 b. The second pixel signal from each pixel 10 output to the second vertical signal line VLb is input to the second column circuit units 40 b 1 to 40 b 3 and converted to digital signals. The noise signal and the pixel signal, which are converted into the digital signals, are input to the signal processing unit via the horizontal transfer unit 60. The signal processing unit performs a correlated double sampling to perform subtraction processing between the noise signal and the addition pixel signal of the pixel 10.

From time t3 to time t5, readout of noise signals and readout of pixel signals from pixels in the middle row are performed in the same manner as in the period from time t1 to time t3. From time t5 to time t7, readout of noise signals and readout of pixel signals from pixels in the lowermost row are performed in the same manner as in the period from time t1 to time t3. In this way, in the first control mode shown in FIG. 6, a signal based on the charge generated by the first photoelectric conversion unit 11 a is read out to the first vertical signal line VLa while a signal based on the charge generated by the second photoelectric conversion unit 11 b is read out to the first vertical signal line VLb.

In a timing chart shown in FIG. 7, the horizontal axis indicates time and also control signals input to units of the image sensor 3 in FIG. 5 in the case of the one-row reading scheme of the second control mode. The vertical drive unit 50 sets signals ADD_FD1<2> to ADD_FD1<0> to high level, signals ADD_FD2<2> to ADD_FD2<0> to low level, and signals ADD_FD3<2> to ADD_FD3<0> to high level, in order to set the second control mode. The signal ADD_FD1 is set to high level, so that the first accumulation unit 14 a and the first amplification unit 15 a of each pixel 10 are electrically connected to each other. The signal ADD_FD2 is set to low level, so that the second accumulation unit 14 b and the second amplification unit 15 b of each pixel 10 are electrically disconnected from each other. Additionally, the signal ADD_FD3 is set to high level, so that the first accumulation unit 14 a and the first accumulation unit 14 b of each pixel 10 are electrically connected to each other.

The vertical drive unit 50 sets the signal CS1_EN to high level and also sets the signal CS2_EN to low level. The signal CS1_EN is set to high level, so that a current is supplied from the first current source 25 a to the first vertical signal line VLa. Additionally, the signal CS2 EN is set to low level, so that the switch unit 31 b of the second current control unit 30 b is turned off and the switch unit 32 b is turned on. Thereby, supply of the current from the second current source 25 b to the second vertical signal line VLb is stopped, and a voltage Vclip is supplied via the switch unit 32 b. The second vertical signal line VLb is fixed at a predetermined voltage to avoid a floating state.

At time t1 shown in FIG. 7, the signal RS<2> is set to high level, so that the transistors M2 a, M2 b of the first and second reset units 13 a, 13 b are turned on in each of the pixels 10 in the uppermost row. Potentials of the first accumulation unit 14 a, the second accumulation unit 14 b, the first region 20 a, and the second region 20 b become reset potentials. In this case, since the first accumulation unit 14 a, the second accumulation unit 14 b, and the first region 20 a of the pixel 10 are connected to one another, potentials of the first and second accumulation units 14 a, 14 b and the first region 20 a are averaged.

Further, at time t1, the signal SEL1<2> is set to high level, so that a signal based on the reset potential is output to the first vertical signal line VLa by the first amplification unit 15 a and the first selection unit 16 a. That is, a noise signal generated when the potentials of the first and second accumulation units 14 a, 14 b and the first region 20 a are reset to the reset potentials is read out to the first vertical signal line VLa. The noise signal from each pixel 10 output to the first vertical signal line VLa is input to the first column circuit units 40 a 1 to 40 a 3 and converted to digital signals.

At time t2, the signal TX<2> is set to high level, so that the transistors M1 a, M1 b of the first and second transfer units 12 a, 12 b are turned on. As a result, the charge obtained through photoelectric conversion by the first and second photoelectric conversion units 11 a, 11 b is transferred to the first and second accumulation units 14 a, 14 b and the first region 20 a.

Further, at time t2, since the signal SEL1<2> is set to high level, an addition pixel signal is output to the first vertical signal line VLa by the first amplification unit 15 a and the first selection unit 16 a. The addition pixel signal from each pixel 10 output to the first vertical signal line VLa is input to the first column circuit units 40 a 1 to 40 a 3 and converted to digital signals. The noise signal and the addition pixel signal, which are converted into the digital signals, are input to the signal processing unit via the horizontal transfer unit 60. The signal processing unit performs a correlated double sampling to perform subtraction processing between the noise signal and the addition pixel signal of the pixel 10.

From time t3 to time t5, readout of noise signals and readout of addition pixel signals from pixels in the middle row are performed in the same manner as in the period from time t1 to time t3. From time t5 to time t7, readout of noise signals and readout of addition pixel signals from pixels in the lowermost row are performed in the same manner as in the period from time t1 to time t3. In this way, in the one-row reading scheme shown in FIG. 7, the pixels 10 are sequentially selected row by row. Signals from the two photoelectric conversion units of the pixels 10 are then added to read out the addition pixel signal to the first vertical signal line VLa. Additionally, by stopping the supply of the current from the second current source 25 b, the power consumption of the image sensor 3 can be reduced.

In a timing chart shown in FIG. 8, the horizontal axis indicates time and also control signals input to units of the image sensor 3 in FIG. 5 in the case of the two-row simultaneous reading scheme of the second control mode. The vertical drive unit 50 sets signals ADD_FD1<2> and ADD_FD1<0> to high level, and a signal ADD_FD1<1> to low level. Further, the vertical drive unit 50 sets signals ADD_FD2<2> and ADD_FD2<0> to low level, and a signal ADD_FD2<1> to high level. Furthermore, the vertical drive unit 50 sets signals ADD_FD3<2> to ADD_FD3<0> to high level. The signal ADD_FD3 is set to high level, so that the first accumulation unit 14 a and the first accumulation unit 14 b of each pixel 10 are electrically connected to each other.

In each of the pixels 10 in the uppermost row and the lowermost row, the signals ADD_FD1<2> and ADD_FD1<0> are set to high level, so that the first accumulation unit 14 a and the first amplification unit 15 a are electrically connected to each other. Additionally, the signals ADD_FD2<2> and ADD_FD2<0> are set to low level, so that the second accumulation unit 14 b and the second amplification unit 15 b are electrically disconnected from each other.

In each pixel 10 in the central row, the signal ADD_FD1<1> is set to low level, so that the first accumulation unit 14 a and the first amplification unit 15 a are electrically disconnected from each other. Additionally, the signal ADD_FD2<1> is set to high level, so that the second accumulation unit 14 b and the second amplification unit 15 b are electrically connected to each other.

The vertical drive unit 50 sets the signal CS1_EN to high level and also sets the signal CS2_EN to high level. Thus, a current is supplied from the first current source 25 a to the first vertical signal line VLa, and a current is supplied from the second current source 25 b to the second vertical signal line VLb.

At time t1 shown in FIG. 8, the signal RS<2> is set to high level, so that the transistors M2 a, M2 b of the first and second reset units 13 a, 13 b are turned on in each of the pixels 10 in the uppermost row. Potentials of the first accumulation unit 14 a, the second accumulation unit 14 b, the first region 20 a, and the second region 20 b become reset potentials. In this case, since the first accumulation unit 14 a, the second accumulation unit 14 b, and the first region 20 a of the pixel 10 are connected to one another, potentials of the first and second accumulation units 14 a, 14 b and the first region 20 a are averaged.

Further, at time t1, the signal SEL1<2> is set to high level, so that a signal based on the reset potential of the pixel 10 in the uppermost row is output to the first vertical signal line VLa by the first amplification unit 15 a and the first selection unit 16 a. That is, a noise signal generated when the potentials of the first and second accumulation units 14 a, 14 b and the first region 20 a are reset to the reset potentials is read out to the first vertical signal line VLa. The noise signal from each pixel 10 in the uppermost row output to the first vertical signal line VLa is input to the first column circuit units 40 a 1 to 40 a 3 and converted to digital signals.

Further, at time t1, the signal RS<1> is set to high level, so that the transistors M2 a, M2 b of the first and second reset units 13 a, 13 b are turned on in each of the pixels 10 in the middle row. Potentials of the first accumulation unit 14 a, the second accumulation unit 14 b, the first region 20 a, and the second region 20 b become reset potentials. In this case, since the first accumulation unit 14 a, the second accumulation unit 14 b, and the second region 20 b of the pixel 10 are connected to one another, potentials of the first and second accumulation units 14 a, 14 b and the second region 20 b are averaged.

Further, at time t1, the signal SEL2<1> is set to high level, so that a signal based on the reset potential of the pixel 10 in the middle row is output to the second vertical signal line VLb by the second amplification unit 15 b and the second selection unit 16 b. That is, a noise signal generated when the potentials of the first and second accumulation units 14 a, 14 b and the second region 20 b are reset to the reset potentials is read out to the second vertical signal line VLb. The noise signal from each pixel 10 in the middle row output to the second vertical signal line VLb is input to the second column circuit units 40 b 1 to 40 b 3 and converted to digital signals.

At time t2, the signal TX<2> is set to high level, so that the transistors M1 a, M1 b of the first and second transfer units 12 a, 12 b are turned on in the pixel 10 in the uppermost row. Thus, the charge obtained through photoelectric conversion by the first and second photoelectric conversion units 11 a, 11 b is transferred to the first and second accumulation units 14 a, 14 b and the first region 20 a.

Further, at time t2, since the signal SEL1<2> is set to high level, an addition pixel signal of the pixel 10 in the uppermost row is output to the first vertical signal line VLa by the first amplification unit 15 a and the first selection unit 16 a. The addition pixel signal from each pixel 10 in the uppermost row output to the first vertical signal line VLa is input to the first column circuit units 40 a 1 to 40 a 3 and converted to digital signals.

At time t2, the signal TX<2> is set to high level, so that the transistors M1 a, M1 b of the first and second transfer units 12 a, 12 b are turned on in the pixel 10 in the middle row. Thus, the charge obtained through photoelectric conversion by the first and second photoelectric conversion units 11 a, 11 b is transferred to the first and second accumulation units 14 a, 14 b and the second region 20 b.

Further, at time t2, when the signal SEL2<1> is set to high level, an addition pixel signal of the pixel 10 in the middle row is output to the second vertical signal line VLb by the second amplification unit 15 b and the second selection unit 16 b. The addition pixel signal from each pixel 10 in the middle row output to the second vertical signal line VLb is input to the second column circuit units 40 b 1 to 40 b 3 and converted to digital signals. The noise signal and the addition pixel signal, which are converted into the digital signals, are input to the signal processing unit via the horizontal transfer unit 60. The signal processing unit performs a correlated double sampling to perform subtraction processing between the noise signal and the addition pixel signal of the pixel 10.

In a period after time t3, simultaneous readouts from pixels in an odd number row and an even number row which are adjacent to each other are performed sequentially, as in the period from time t1 to time t3. In this way, in the two-row simultaneous readout scheme shown in FIG. 8, signals of pixels in two rows can be simultaneously read out. A signal can thus be read out at a high speed from each pixel 10 arranged in the image sensor 3.

The following description represents how to select and use the first control mode, the one-row reading scheme of the second control mode, and the two-row simultaneous reading scheme of the second control mode. In a case where the camera 1 performs the focus adjustment operation, the control unit 4 controls the image sensor 3 in the first control mode. Further, in a case where the camera 1 displays a through image (live view image) of a subject on the display unit 6, the control unit 4 controls the image sensor 3 in one-row reading scheme or two-row simultaneous reading scheme of the second control mode. Thus, in a case where the camera 1 displays a through image (live view image) of a subject on the display unit 6 while performing the focus adjustment operation, the control unit 4 controls the image sensor 3 in a time division manner in the first control mode and the second control mode with the one-row reading scheme or the two-row simultaneous reading scheme. In a case where a release operation member of the operation unit 7 is operated, the control unit 4 controls the image sensor 3 in the one-row reading scheme or the two-row simultaneous reading scheme of the second control mode.

Furthermore, in a case where the camera 1 performs photographing with a high frame rate, e.g., high-speed continuous photographing or moving image photographing, the control unit 4 controls the image sensor 3 in the two-row simultaneous reading scheme of the second control mode for a high-speed readout of addition pixel signals. Additionally, when the subject movement speed detection unit provided in the camera 1 detects that a subject is moving at a relatively high speed, the control unit 4 also controls the image sensor 3 in the two-row simultaneous reading scheme of the second control mode in order to read out addition pixel signals at a high speed to reduce blurring of the image. On the other hand, when a battery level detection unit detects that a remaining amount of a drive battery of the camera 1 is reduced, the control unit 4 controls the image sensor 3 in the one-row reading scheme of the second control mode in order to reduce a consumption of the battery.

According to the embodiment described above, the following advantageous effects can be achieved.

(1) The image sensor 3 includes a pixel 10 having: a first photoelectric conversion unit 11 a that photoelectrically converts incident light to generate a first charge; a second photoelectric conversion unit 11 b that photoelectrically converts incident light to generate a second charge; a first accumulation unit 14 a that accumulates the first charge generated by the first photoelectric conversion unit 11 a; a second accumulation unit 14 b that accumulates the second charge generated by the second photoelectric conversion unit 11 b; a first switch (addition switch unit 17) that connects and disconnects the first accumulation unit 14 a to/from the second accumulation unit 14 b; a first output unit (a first amplification unit 15 a and a first selection unit 16 a) that is connected to the first accumulation unit 14 a; and a second output unit (a second amplification unit 15 b and a second selection unit 16 b) that is connected to and disconnected from the second accumulation unit 14 b via a second connection unit (a second connection switch unit 19 b). In the present embodiment, when the transistor M9 b of the second connection switch unit 19 b is turned off, the second region 20 b is electrically disconnected from the first and second accumulation units 14 a, 14 b, the first region 20 a, and the first amplification unit 15 a. Thus, the conversion gain of the charge voltage can be increased. Consequently, a S/N ratio can be improved to obtain an addition pixel signal having a lower noise.

(2) In the present embodiment, when the transistor M9 b of the second connection switch unit 19 b is turned off, the first amplification unit 15 a is electrically disconnected from the gate capacitance of the transistor M3 b of the second amplification unit 15 b. It is therefore possible to prevent the conversion gain from fluctuating under the influence of fluctuation of the gate capacitance of the transistor M3 b. As a result, an addition pixel signal having a high linearity can be obtained.

The following modifications are also included within the scope of the present invention, and one or more of the modifications may be combined with the above-described embodiment.

First Modification

In the embodiment described above, an example has been described in which the first connection switch unit 19 a and the second connection switch unit 19 b are provided in the pixel 10. However, the pixel may be configured to have only one of the first connection switch unit 19 a and the second connection switch unit 19 b. As shown in FIG. 9, in the image sensor 3, a pixel 10A having a first connection switch unit 19 a and a pixel 10B having a second connection switch unit 19 b may be arranged.

The transistor M9 a of the first connection switch unit 19 a of the pixel 10A is turned off, so that the first region 20 a is electrically disconnected from the first and second accumulation units 14 a, 14 b and the second region 20 b. The conversion gain of the first and second accumulation units 14 a, 14 b and the second region 20 b can therefore be increased. On the other hand, the transistor M9 b of the second connection switch 19 b of the pixel 10B is turned off, so that the second region 20 b is electrically disconnected from the first and second accumulation units 14 a, 14 b and the first region 20 a. The conversion gain of the first and second accumulation units 14 a, 14 b and the second region 20 b can therefore be increased. In the example shown in FIG. 9, for example, for pixels in two rows, addition pixel signals are read out from a pixel 10A in one row to the second vertical signal line VLb and, at the same time, addition pixel signals are read out from a pixel 10B in the other row to the first vertical signal line VLa.

Second Modification

In the embodiment described above, an example has been described in which a plurality of photoelectric conversion units are arranged for each pixel 10. However, the pixel 10 may be configured to have one photoelectric conversion unit. In this case, for example, as shown in FIG. 10, the pixel 10 is provided with the addition switch unit 17 and the connection switch unit 19. The addition switch unit 17 connects the accumulation units 14 of the plurality of pixels 10 arranged in the row direction, for example. The connection switch unit 19 connects the accumulation unit 14 to the amplification unit 15. The control unit 4 can separate the accumulation unit 14 and the amplification unit 15 of the pixel 10 by performing an on/off control of the connection switch unit 19 so that the conversion gain of the charge voltage can be increased. Note that the connection switch unit 19 may not be arranged for each pixel 10. In the image sensor 3, pixels having the connection switch unit 19 and pixels not having the connection switch unit 19 may be arranged. For example, in the image sensor 3, pixels having the connection switch unit 19 and pixels not having the connection switch unit 19 may be alternately arranged. Further, the addition switch unit 17 may not be arranged for each pixel 10. Each addition switch unit 17 may be arranged for a plurality of pixels and shared by the plurality of pixels.

Third Modification

In the embodiment described above, an example has been described in which two photoelectric conversion units are arranged in one pixel; however, the configuration of the pixel is not limited thereto. The pixel may be configured to have three or more photoelectric conversion units for one pixel. In this case, for example, signals from the plurality of photoelectric conversion units may be individually read out in the first control mode, and signals from two or more photoelectric conversion units among the plurality of photoelectric conversion units may be added and read out in the second control mode.

Fourth Modification

In the embodiment and the modifications described above, an example has been described in which photodiodes are used as the photoelectric conversion units. However, a photoelectric conversion film may be used as the photoelectric conversion unit.

Fifth Modification

The image sensor 3 described in the embodiment and the modifications described above may be applied to a camera, a smartphone, a tablet, a camera built in a PC, an on-vehicle camera, a camera mounted on an unmanned aerial vehicle (drone, radio control machine, etc.).

Although various types of embodiments and modifications have been described above, the present invention is not limited to these. Other aspects contemplated within the scope of the technical idea of the present invention are also included within the scope of the present invention.

The disclosure of the following priority application is herein incorporated by reference:

Japanese Patent Application No. 2017-16284 (filed on Jan. 31, 2017)

REFERENCE SIGNS LIST

3 . . . image sensor, 4 . . . control unit, 10 . . . pixel, 11 a . . . first photoelectric conversion unit, 11 b . . . second photoelectric conversion unit, 17 . . . addition switch unit, 19 a . . . first connection switch unit, 19 b . . . second connection switch unit, 50 . . . vertical drive unit. 

The invention claimed is:
 1. An image sensor comprising a pixel, the pixel having: a first photoelectric conversion unit and a second photoelectric conversion unit that photoelectrically convert light to generate a charge; an accumulation unit that performs at least one of the following: (i) accumulation via a first transfer unit of the charge generated by the first photoelectric conversion unit and (ii) accumulation via a second transfer unit of the charge generated by the second photoelectric conversion unit; a first output unit and a second output unit that output a signal based on the charge accumulated in the accumulation unit; and an adjustment unit that reduces a capacitance of the accumulation unit if a signal based on the charges generated by the first photoelectric conversion unit and the second photoelectric conversion unit is output from the first output unit.
 2. The image sensor according to claim 1, wherein: the accumulation unit has a first accumulation unit that accumulates via the first transfer unit the charge generated by the first photoelectric conversion unit and a second accumulation unit that accumulates via the second transfer unit the charge generated by the second photoelectric conversion unit; the first output unit is connected to the first accumulation unit; the second output unit is connected to the second accumulation unit; and the adjustment unit reduces a total capacitance formed by the first accumulation unit and the second accumulation unit if the signal based on the charges generated by the first photoelectric conversion unit and the second photoelectric conversion unit is output from the first output unit.
 3. The image sensor according to claim 2, wherein: the second output unit and the second accumulation unit are connected by a second connection unit, and the adjustment unit reduces the total capacitance of the accumulation unit by the second connection unit if the signal based on the charges generated by the first photoelectric conversion unit and the second photoelectric conversion unit is output from the first output unit.
 4. The image sensor according to claim 1, wherein the first transfer unit and the second transfer unit are a first transfer means and a second transfer means, respectively.
 5. The image sensor according to claim 3, wherein; if the signal based on the charges generated by the first photoelectric conversion unit and the second photoelectric conversion unit is output from the first output unit, the adjustment unit disconnects the second connection unit so that the second output unit is disconnected from the second accumulation unit.
 6. The image sensor according to claim 1, wherein the first transfer unit and the second transfer unit comprise a first transistor and a second transistor, respectively.
 7. The image sensor according to claim 3, wherein: the adjustment unit reduces a part of capacitance of the second accumulation unit by the second connection unit if the signal based on the charges generated by the first photoelectric conversion unit and the second photoelectric conversion unit is output from the first output unit.
 8. The image sensor according to claim 5, wherein: the first output unit and the first accumulation unit are connected by a first connection unit; the adjustment unit adjusts the total capacitance of the accumulation unit by the first connection unit; and if the signal based on the charges generated by the first photoelectric conversion unit and the second photoelectric conversion unit is output from the first output unit, the adjustment unit connects the first connection unit so that the first output unit is connected to the first accumulation unit.
 9. An image sensor comprising a pixel, the pixel having: a first photoelectric conversion unit and a second photoelectric conversion unit that photoelectrically convert light to generate a charge; a first accumulation unit that accumulates via a first transfer unit the charge generated by the first photoelectric conversion unit; a second accumulation unit that accumulates via a second transfer unit the charge generated by the second photoelectric conversion unit; a first output unit that is connected to the first accumulation unit via a first connection unit; a second output unit that is connected to the second accumulation unit via a second connection unit; a third connection unit that connects the first accumulation unit to the second accumulation unit; a first signal line that is connected to the first output unit and to which a signal is output from the first output unit; and a second signal line that is connected to the second output unit and to which a signal is output from the second output unit.
 10. The image sensor according to claim 9, wherein: if a signal based on the charge generated by the first photoelectric conversion unit and a signal based on the charge generated by the second photoelectric conversion unit are output from the first output unit to the first signal line, the second connection unit disconnects the second output unit from the second accumulation unit.
 11. The image sensor according to claim 9, wherein: if a signal based on the charge accumulated by the first accumulation unit and a signal based on the charge accumulated by the second accumulation unit are output from the first output unit to the first signal line, the second connection unit disconnects the second output unit from the second accumulation unit.
 12. The image sensor according to claim 10, wherein: if the signal based on the charge generated by the first photoelectric conversion unit and the signal based on the charge generated by the second photoelectric conversion unit are output from the first output unit to the first signal line, the first connection unit connects the first accumulation unit to the first output unit.
 13. The image sensor according to claim 11, wherein: if the signal based on the charge accumulated by the first accumulation unit and the signal based on the charge accumulated by the second accumulation unit are output from the first output unit to the first signal line, the first connection unit connects the first accumulation unit to the first output unit.
 14. The image sensor according to claim 9, wherein if the first accumulation unit and the second accumulation unit are connected by the third connection unit, a signal based on the charge accumulated in the first accumulation unit and a signal based on the charge accumulated in the second accumulation unit are output from the first output unit to the first signal line.
 15. The image sensor according to claim 14, wherein: if the first accumulation unit and the second accumulation unit are disconnected by the third connection unit, the signal based on the charge accumulated in the first accumulation unit is output from the first output unit to the first signal line, and the signal based on the charge accumulated in the second accumulation unit is output from the second output unit to the second signal line.
 16. The image sensor according to claim 9, wherein: the first transfer unit includes a first transfer transistor that transfers the charge generated by the first photoelectric conversion unit to the first accumulation unit; the second transfer unit includes a second transfer transistor that transfers the charge generated by the second photoelectric conversion unit to the second accumulation unit; the first accumulation unit is a drain of the first transfer transistor; the second accumulation unit is a drain of the second transfer transistor; the first output unit is a first amplification transistor that outputs a signal based on the charge generated by the first photoelectric conversion unit; the second output unit is a second amplification transistor that outputs a signal based on the charge generated by the second photoelectric conversion unit; the first connection unit connects the first accumulation unit to the first amplification transistor; the second connection unit connects the second accumulation unit to the second amplification transistor; if the signal based on the charge generated by the first photoelectric conversion unit and the signal based on the charge generated by the second photoelectric conversion unit are output from the first output unit to the first signal line, the charge generated by the first photoelectric conversion unit and the charge generated by the second photoelectric conversion unit are accumulated in the first accumulation unit and the second accumulation unit and a gate of the first amplification transistor; and if the signal based on the charge generated by the first photoelectric conversion unit is output from the first output unit to the first signal line and the signal based on the charge generated by the second photoelectric conversion unit is output from the second output unit to the second signal line, the charge generated by the first photoelectric conversion unit is accumulated in the first accumulation unit and a gate of the first amplification transistor, and the charge generated by the second photoelectric conversion unit is accumulated in the second accumulation unit and a gate of the second amplification transistor.
 17. The image sensor according to claim 9, wherein: a plurality of the pixels are arranged in a first direction and the plurality of the pixels has a first pixel and a second pixel; the first output unit of the first pixel outputs a signal based on the charges generated by the first and second photoelectric conversion units to the first signal line; and the second output unit of the second pixel outputs a signal based on the charges generated by the first and second photoelectric conversion units to the second signal line.
 18. The image sensor according to claim 17, wherein: in the first pixel, if the signal based on the charges generated by the first and second photoelectric conversion units is output from the first output unit to the first signal line, the first connection unit connects the first accumulation unit to the first output unit and the second connection unit disconnects the second output unit from the second accumulation unit; and in the second pixel, when the signal based on the charges generated by the first and second photoelectric conversion units is output from the second output unit to the second signal line, the first connection unit disconnects the first output unit from the first accumulation unit and the second connection unit connects the second accumulation unit to the second output unit.
 19. An electronic camera, comprising: the image sensor according to claim 1; and an image generation unit that generates image data based on a signal output from the image sensor.
 20. An electronic camera, comprising: the image sensor according to claim 9; and an image generation unit that generates image data based on a signal output from the image sensor. 